1. Field of the Invention
The present invention relates to an endoscope system in which an endoscope for transmitting an image by fibers is provided with a low-pass filter for moire removal and which can use a common TV camera together with the endoscope which transmits the image by a lens or lenses.
2. Description of the Related Art
Endoscopes, particularly hard endoscopes, can be divided into two types including a lens relay type which uses the lenses consisting of a homogeneous or heterogeneous medium in an image transmitting optical system, and a fiber relay type which uses an image guide (hereinafter referred to as "IG"). In the case where high resolution is required, image transmission due to the lenses has been conducted. At this time, an optical low-pass filter (hereinafter referred to as "LPF") which has, normally, a double refraction plate of a degree of a single plate is inserted into an optical path of the TV camera which is used in combination with the hard endoscope, in order that the resolution is not injured, but only a pseudocolor is removed. Meanwhile, in the case where reduction in diameter is required for the hard endoscope, an IG is used. At this time, about three.about.four LPFs are inserted into the optical path of the TV camera which is used in combination with the hard endoscope, in order to reduce the moire which is generated by interference in cyclic structure or construction between the IG and an electronic image-pickup element (hereinafter referred to as "CCD").
In this manner, in the prior art, the TV camera which is provided with different LPFs has been used in accordance with the type of the image transmission optical system of the used endoscope.
Over against this, as an arrangement in which, even in case where the image transmission optical systems are different in type from each other, as the TV camera can commonly be used, there is an apparatus which is disclosed in Japanese Patent Unexamined Publication No. HEI 1-284225 (284225/1989). The apparatus is arranged such that LPFs each of which is formed by two.about.six quartz plates and which have different characteristics are prepared, and the LPFs which are inserted into the optical path of the TV camera in accordance with the combined endoscope can selectively be exchanged.
In this manner, the reason as to why the different LPFs are used in accordance with the type of the image transmission optical system is as follows:
The type which uses the relay lenses is arranged such that an image of an object is image-formed on the image pickup element by the lenses. Accordingly, the image of the object directly interferes with an array of picture elements or pixels of the image pickup element. Since, however, the frequency in which a fine frequency component similar to the repeating period of the picture element of the image pickup element is included in the object is relatively low, the generation of moire in the image is remarkably low. On the contrary, the arrangement which uses the image guide is such that an end face of the IG is imaged on the image pickup element. Since, however, the IG is an arrangement in which a multiplicity of fibers are bundled together, the IG includes such a plurality of frequency components that the IG interferes with the arrangement of the picture elements of the image pickup element. Specifically, the fibers have a core which transmits a light and a clad which does not transmit the light. Accordingly, the image on the end face of the fiber bundle in which the multiplicity of fibers are bundled is made to one in which a plenty of cores closely are arranged. The repeating patterns of the core interfere with the arrangement of the picture elements of the image pickup element, and remarkable moire is generated in the TV image. Accordingly, the different LPFs are required for the arrangement which uses the relay lenses and for the arrangement which uses the IG, in order to remove the moire.
By the way, there are various kinds of LPFs. However, a typical LPF has a double refraction plate such as a quartz plate or the like.
The double refraction plate has a property thereof which separates an incident light into a normal light and an abnormal light to output the same. Accordingly, if the double refraction plate is arranged at an adequate position in an image pickup light path, a double image of the object is formed on an image plane. An optical element for forming the double image effects to limit spatial frequency with respect to a direction in which the image is shifted. Specifically, the array having the repetitive period corresponding to an amount of shift of the two images is eliminated with reference to the direction in which the image is shifted. Accordingly, in the shifted direction, the spatial frequency response of the imaging optical system which includes the LPF becomes zero in the spatial frequency in accordance with the amount of shift of the image. The arrangement is such that such effects which limit the spatial frequency is utilized whereby the required number of quartz plates is arranged in a TV image pickup optical system, and an image separation direction of each plate is set adequately, to remove the moire.
In this case, the effects which limit the spatial frequency of the LPF are generated or occur only in a direction which separates a light ray retarding a single double-refraction plate. Accordingly, even if the same number of double refraction plates is combined with each other, the spatial frequency characteristics of the LPF are different from each other, depending upon how to set the light-ray separating directions. Thus, when the LPF is designed adequately, the number of double refraction plates and the light-ray separation direction are selected in accordance with the pattern of the arrangement of the picture element of the solid-state image pickup element, such as a CCD or the like, or the arrangement of the fibers of the IG.
Generally, a countermeasure against the moire which is generated by the interference between the IF and the CCD is dealt with by the fact that the LPF is provided on the side of the TV camera. A separation direction of a point due to the LPF has been designed on the assumption that the positional relationship which is fixed with respect to a scanning direction on a CCD face or plane is held or retained.
In recent years, diffusion or popularization of the surgery operations which use endoscopes abruptly increases the use of hard endoscopes in a surgical range or region. Further, almost all of the observation configuration or form in the range of surgery is image observation due to the TV monitor. Moreover, there are frequently forced the necessities that, during the operation, the hard endoscope of lens relay type which transmits the image by the lenses and the hard endoscope of fiber relay type which transmits the image by the fibers are selectively used. For this reason, in recent years, there has been an increased need for endoscope systems which can be used in such a manner that the aforesaid both types of hard endoscopes are connected to the same TV camera increase.
In the case where the LPFs of a degree of three.about.four are inserted into the optical path of the TV camera, the hard endoscope of lens relay type causes a decrease of resolution thereof. Meanwhile, in the case where only the LPF having a degree of one is inserted into the optical path of the TV camera, the moire is generated when used in combination with the hard endoscope of fiber relay type.
In an apparatus disclosed in Japanese Patent Unexamined Publication No. HEI 1-284225 (284225/1989), the LPF on the side of the TV camera must be replaced each time the hard endoscope is replaced, which causes problems when in use. In a surgical operation, many treatment tools must be treated or handled other than the endoscope. Thus, the prompt countermeasure must be taken during the operation. Accordingly, all actions which are time-consuming must be removed or excluded.
At present, however, it is extremely difficult to use the same TV camera between the endoscopes which have different image transmitting structures. Furthermore, a device which can realize the same has not yet been demonstrated.
Further, there is a need for observation, in the surgical operation, of the side of the abdomen or bauchseite which is always displayed correspondingly to an upper part of an image plane of a monitor. Particularly, in the case of the hard endoscope which observes a strabismus direction with respect to an insertion direction, the observation must be carried out while the scope is rotated during the operation. Along with this, the TV camera is frequently rotated to frequently conduct positional adjustment of the bauchseite which is displayed on the monitor. Specifically, for the aforesaid reason, the TV camera is universally rotated and is used with respect to a body of the endoscope, during the operation.
Meanwhile, since a conventional soft endoscope has a function of a wire angle or the like which can universally remote-operate the direction of a distal end thereof, the endoscope body has not been required to be rotated in the case of the strabismus. In this manner, it is peculiar to the hard endoscope to require the rotation of the endoscope body. Since the IG itself has flexibility, the IG has been mainly used in the soft endoscope, and it has been rare that the IG has been used in the hard endoscope. For this reason, it is not assumed that the hard endoscope of the fiber relay type is universally rotated and is used with respect to the TV camera, and this problem has conventionally not been considered. Accordingly, in the prior art, the relative positional relationship between the scanning direction on the CCD plane and the point separation direction of the LPF has been considered fixed. Any device on the assumption that the scanning direction is rotated universally has not yet been demonstrated.
In this manner, the endoscope system in which observation is conducted by the common TV camera, the TV camera can universally be rotated, and the moire is sufficiently suppressed in an optional rotational direction of the TV camera has not yet been demonstrated.
Although the conception is different, as an arrangement which is the closest to the arrangement of the endoscope system according to the present embodiment in an enforcement mode, there is an apparatus which is disclosed in JP-59-193416, corresponding to U.S. Pat. No. 4,676,593. The apparatus is arranged such that, in order not to make prominent the mesh or network construction of the IG, the LPF having three quartz plates is provided in an ocular optical system, and the separation directions of the point due to the quartz plate are joined to each other every shifting of 60.degree. in accordance with a transverse row or line (hexagonal density) of loading of the fiber.
However, in this apparatus, since the separation direction of the point is shifted every 60.degree., the separation strength of the point due to the quartz plate comes into 1/3, and is not uniform. For this reason, the sufficient effects of moire removal cannot be acquired. Further, since it is not assumed that the TV camera is rotated, it is impossible to suppress the moire in an optional rotational direction. Moreover, consideration is not also paid to the adaptability of the TV camera.
Furthermore, as will be described on and after a tenth embodiment, the optical device or the image pickup device such as the endoscope device or the like in which the quartz plate is used to remove the moire increases in cost.